Embodiments of the present invention relate to the generation of a pattern on a substrate using electron beams.
An electron beam pattern generator typically comprises an electron beam column in which a single electron beam is generated, modulated, and directed onto a substrate, to expose an electron-sensitive material on the substrate. An electron beam source and modulator generates the electron beam and modulates its intensity. Beam optics focus the electron beam and a beam scanning system scans the electron beam across the substrate. Such pattern generators are used to generate electron beam patterns on electron sensitive resist on substrates, such as a semiconductor wafers and masks.
Conventional pattern generators that use a single electron beam for writing an electron beam pattern on a substrate do not provide a sufficiently high throughput when writing high resolution patterns. A single electron beam can write an electron pattern only at relatively slow speeds across the substrate. For example, at current line width resolutions of 100 to 130 nm, a single electron beam system takes about 6 hours to write a pattern on a 200 mm substrate; however, at resolutions of 35 to 50 nm, the same system would take about 50 hours to write the pattern—which is too long. Thus, single beam systems have limited throughput at high resolutions.
Multiple electron beam pattern generators, which use a plurality of electron beams to generate an electron beam pattern on a substrate, can provide higher throughput and speed even at high resolutions. Multiple electron beams may be drawn from one or more electron sources as separate and well-defined beams. In one method, the multiple electron beams are generated by a photocathode-laser system that directs a modulated laser beam at a particular wavelength or frequency on a photocathode to emit the electron beams. The photocathode comprises a photoemissive material that emits modulated electron beams when illuminated by the laser beam pulses. The laser beam source and optics are selected to generate a laser beam having an energy of at least the workfunction of the photoemissive material to be able to excite the electrons to a suitable energy level when photons of the laser beam impinge on the photoemissive material. Thus, the properties of the photoemissive material and the laser beam generator need to be matched to operate suitably in combination.
However, conventional photocathode-laser systems have several problems. For example, one photocathode-laser system combination comprises an argon-ion laser source, a frequency multiplier crystal, and a photocathode comprising an Mg, MgO or CsTe based photoemissive material. The argon-ion laser source is desirable because it is well characterized, commercially available, and can be frequency matched to energize Mg and MgO photocathodes. One of the fundamental wavelengths of the argon-ion laser is 514 nm. The frequency multiplier crystal reduces the wavelength to about 257 nm to achieve a laser beam having an energy level of about 4.8 eV which is desirable. The frequency multiplied laser beam, with the energy of 4.8 eV, has a higher energy than the workfunction of Mg or MgO photoemissive material, which is 3 to 4 eV, thus the laser system and the photocathode are suitably matched. However, Mg and MgO photocathodes are limited in their current yield by problems such as oxidation and degradation. Oxidation of a Mg photocathode occurs during low pressure operation due to residual oxygen in the chamber and gives rise to deleterious blanking effects that manifest as a change of photoemission when the laser beam is blanked, i.e., turned “on” after an “off” period of a few minutes. MgO photocathodes can also give rise to such deleterious blanking effects. Mg photocathodes also degrade when the material deteriorates over a time period after a few hours of operation in vacuum systems with pressures of, for example, about 1×10−10 Torr. Other photocathode materials such as CsTe are also often subject to growth of the emission spot during operation and require patterned cathodes.
Thus, it is desirable to have a better and more consistently performing matched photocathode-laser system that is capable of generating multiple electron beams. It is also desirable ensure that the photocathode-laser system is properly matched. It is further desirable to have a photocathode that is stable and can generate electron beams without excessive degradation over time due to oxidation or from being in a vacuum environment. It is also desirable to have an electron beam pattern generating system capable of providing high throughput at good resolutions.